Determination of Hydrogen-Atom Concentration by Lyman-α Photometry. I. Oscillator Strength of the Hydrogen-Atom 2P,½←2S½ Transition. II. Kinetics of the Reaction of Hydrogen Atoms with Acetylene and Ethylene

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Abstract
A photometric method of determining hydrogen‐atom concentration in the gas phase has been developed. It consists of a hydrogen—neon lamp emitting Lyman‐α radiation at 1216 Å, and a nitric oxide‐filled ion chamber which serves as the detector. A fast‐flow system with a microwave discharge in an H2–He mixture was used as the source of hydrogen atoms. This arrangement permits the determination of H‐atom concentrations of 1011 atoms/cc. A photometric calibration curve was obtained by using titration with NO2 as an absolute measure of H‐atom concentration. The oscillator strength of the Lyman‐α transition was determined with this apparatus. Depending on the assumed profile of the emission line and the extent of self‐reversal in the source, oscillator strengths of 0.4 to 1.1 (compared with the theoretical value of 0.8324) were obtained. Rates of reaction of H atoms with ethylene and acetylene were determined at various total pressures and concentrations of reactants. Both reactions are first order with respect to each reactant, and the recombination of H atoms contributes very little to their rate of disappearance. The pressure effect on the reaction with ethylene depends on the formation of vibrationally excited ethyl radicals which either decompose to form reactants or are stabilized by collision. The reaction with acetylene appears to be complicated by a pressure‐independent reaction path which predominates at low pressures and is most probably the abstraction reaction H+C2H2→H2+C2H.